Phase Of The Moon N. Used Humorously As A Random Parameter On Which Something Is Said To Depend.

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phase of the moon n.

Used humorously as a random parameter
on which something is said to depend. Sometimes implies
unreliability of whatever is dependent, or that reliability seems
to be dependent on conditions nobody has been able to determine.
"This feature depends on having the channel open in mumble mode,
having the foo switch set, and on the phase of the moon." See
also heisenbug.

True story: Once upon a time there was a program bug that
really did depend on the phase of the moon. There was a little
subroutine that had traditionally been used in various programs at
MIT to calculate an approximation to the moon's true phase. GLS
incorporated this routine into a LISP program that, when it wrote
out a file, would print a timestamp line almost 80 characters long.
Very occasionally the first line of the message would be too long
and would overflow onto the next line, and when the file was later
read back in the program would barf. The length of the first
line depended on both the precise date and time and the length of
the phase specification when the timestamp was printed, and so the
bug literally depended on the phase of the moon!

The first paper edition of the Jargon File (Steele-1983) included
an example of one of the timestamp lines that exhibited this bug,
but the typesetter `corrected' it. This has since been
described as the phase-of-the-moon-bug bug.

However, beware of assumptions. A few years ago, engineers of CERN
(European Center for Nuclear Research) were baffled by some errors
in experiments conducted with the LEP particle accelerator. As the
formidable amount of data generated by such devices is heavily
processed by computers before being seen by humans, many people
suggested the software was somehow sensitive to the phase of the
moon. A few desperate engineers discovered the truth; the error
turned out to be the result of a tiny change in the geometry of the
27km circumference ring, physically caused by the deformation of
the Earth by the passage of the Moon! This story has entered
physics folklore as a Newtonian vengeance on particle physics and
as an example of the relevance of the simplest and oldest physical
laws to the most modern science.